Prevention of ion-caused undercutting in xeroradiography



PREVENTION OF ION-CAUSED UNDERCUTTING IN XERGRADIOGRAPHY Robert G. Vyverberg, Pittsford, N. Y., assignor to The Haloid Company, Rochester, N. Y., a corporation of New York This invention relates to xeroradiography and more particularly to a method and arrangement to eliminate undercutting of images that are radiographed. Undercutting of radiographed objects means that the images reproduced appear smaller than their actual sizes because the edges of the image disappear in the radiographic record.

In xeroradiography the plate or element exposed to X-ray or gamma ray or other penetrating radiation'usual ly comprises a conductive metallic backing sheet having a photo-conductive insulating layer or coating, for example, of vitreous or amorphous selenium on one surface. It is conventional to cover or protect the coating from light by a slide plate spaced from the surface of the coating and usually called a dark slide. The plate or element is sensitized by applying an electrostatic charge to the coating, and the sensitized plate or element is then exposed to penetrating radiation with the object to be radiographed appropriately interposed between the radia tion source and the plate. Under influence of the radiation from the source which readily passes through the dark slide, the coating becomes electrically conductive permitting the electrostatic charge thereon to be selectively dissipated in the portions reached by the beams, less dissipation occurring in those portions of the coating shaded by the object being radiographed. In this manner an electrostatic latent image of the radiographed ob ject is formed on the coating. This image may then be developed with an electrostatic material which clings to the electrically-charged portions of the latent image on the coating.

When a radiographic exposure is made the image pictured on the element or plate is either smaller than its actual size, or in some instances is materially weakened around its edges so that it may appear smaller in size. This is not caused by a reduction in size of all dimensions,

but is merely a disappearance of the image at its edges, 1

i. e. these edges are undercut.

The undercutting of xeroradiographic images is caused by two distinct and separate phenomena. One cause of undercutting is the scattering of secondary X-rays from the radiographed object. The second cause is known experimentally to be the result of air ionization produced in the air space between the dark slide and the surface of the photo-conductive coating. The ionization is caused by primary X-rays, secondary X-rays and secondary high speed electrons. This invention concerns itself as a primary object with the substantial elimination of the second mentioned cause of undercutting just described.

Further objects and features of the invention are the production of a simple device for holding the xeroradiographic plate or element and of making required electrical connections to it and the dark slide for reducing or substantially eliminating undercutting caused by air ioni zation.

Other objects and features of the invention are to provide an improved xeroradiographic process whereby undercutting as caused by air ionization occurring during 2,802,948 Patented Aug. 13, 1957 ice radiation exposure is substantially eliminated or material 1y reduced.

Further objects and features of the invention will be come apparent from the following specification and the accompanying drawings wherein:

Fig. 1 is a cross-sectional view of a xeroradiographic element or plate in association with a dark slide and in eludes diagrammatically an electric circuit for practicing the invention; and

Fig. 2 is a plan view of the xeroradiographic member including the dark slide and means for applying a positive potential to the latter.

Referring to the figures in detail, a xeroradiographic member is shown being generally designated 10 and comprising a xeroradiographic plate or element 11 surround ed by and engaging a rigid frame 12 which surrounds and protects the four sides of the plate. In the preferred embodiment, this frame is light tight and is further provided with a dark screen or slide 13 of conductive metal such as aluminum, slidably mounted in the frame 12v as by the grooves 14.

The xeroradiographic plate 11 consists generally of a conductive metal backing plate 15, having on at least one of its surfaces a coating or layer 16 of an insulating photo-conductive or vitreous form of selenium or other material of similar properties. This selenium layer 16 has the property of retaining in the dark an electrostatic charge applied to it as by friction or by a corona discharge and of discharging or dissipating its retained charge when exposed to light, or X-ray, or gamma ray or other penetrating radiations. Photo-conductive material, as used in this specification, is meant to include either material which is normally non-conductive except when exposed to penetrating radiation, for example, of X-rays, or to materials rendered conductive either by such exposure or by exposure to light.

The dark shield or slide 13 is slidable outwardly of the frame 12 to uncover the selenium coating 16 to permit charging of the coating and it fits in light sealing relationship in the grooved frame 12 to prevent undesired discharge of the charged coating 16 by unwanted exposure to light- Under usual xeroradiographic operating technique, the dark slide 13 and the xeroradiographic plate or element 11 are not connected electrically and the only electric field lines of force in the air space 17 between the slide 13 and coating 16 are fringing fields (not shown) from areas of higher charge to areas of lower charge on the selenium surface 16. If an ion pair is produced in the gap or air space 17 as by penetrating radiation, the negative ion I will be attracted to the area of higher charge because of these fringing fields and partially discharge the plate 11. As the negative ions discharge the charged portion, the position of the fringing fields shifts inwardly toward the middle of the still charged portion. This produces undercutting in the sense that the edges of the radiographed image are lost.

The essence of the instant invention is the discovery that this undercutting can be substantially eliminated by applying a large potential across the dark slide 13 and the plate or element 11 in space 17 such that there is an electrostatic field Whose electric lines of force F are normal to the plate 11 and in a direction such that the negative ions I occurring in the air space 17 during radiation exposure will move toward the dark slide 15 and away from the charged selenium surface 16. This requires that the potential on the dark slide be positive relative to the surface 16.

In xeroradiography, prior to exposure, the selenium surface 16 is charged to a potential ranging from 600 to 700 volts. This is usually effected in known manner by a corona electrode (not shown) or by friction or in other known ways. Since, to avoid undercutting, it is necessary to collect negative ions I on the dark slide 13, the poten tial of the slide must be positive relative to the plate or element 11 and its selenium. coating 16. The-range of positive charge required on the dark plate 13 has been found by experiment to be between approximately 750 and 1800 volts positive on the, dark slide 13 with respect to the conductive metal backing member or sheet 15 of the selenium coating 16. At 750 volts, reduction but not complete elimination of ion-caused undercutting occurs. At 1200 volts substantially complete elimination of such undercutting occurs. At higher voltages, for example, 1500 and 1800 volts, no material improvement over a 1200 volt positive charge is apparent.

In order to apply the required positive charge to the dark slide 13, a conductive spring contact member 18 is attached suitably to an insulative carrier member or block 19 made, for example, of an acrylic resin or other suitable insulating material. The member or block 19 is secured to a conductive metallic backing plate 20. The backing plate 20 has a plateau 21 dimensioned to fit within the inner borders of the frame 12 in direct contact with the conductive metallic backing 15 of the xeroradiographic element 11. Spring contact member 13 then electrically engages the conductive dark slide 13. Spring member 18 is connected through a resistor 22, in this embodiment of approximately megohms, to the positive terminal of a D. C. voltage source 23 whose potential is sufficient to provide a positive potential rang ing from 700 to 1800 volts and preferably approximately 1200 volts on the dark slide 13 relative to metal backing sheet 315. Resistor 22 may have any other selected value that will limit current to the dark slide. it may be eliminated in some instances, if desired.

The conductive metallic backing plate 20 is connected to the negative terminal of said source 23 and through its plateau 21 is in electrical contact with conductive backing sheet of the plate or element 11. This establishes the required large potential electrostatic field F across air space 17 and causes negative ions 1 occurring in the air space 17 during radiation exposure to move toward the dark slide 13 and away from the selenium coating 16. This eliminates undercutting that such ions would cause if the dark slide 13 were unchanged.

The thick metallic backing member 2% which is of steel, brass or lead, or other suitable heavy metal, serves not only to provide the negative electrical contact with the plate or element 11 from source 23, but also to eliminate back scattering of rays during radiographic exposure of the plate or element 11. If back scattering is not objectionable for a certain purpose or other ways of eliminating it are employed the member can be omitted.

It is a frequent practice to invert the xerographic member it? so that the plate 11 is uppermost with its layer 16 on its bottom and with the dark slide 13 below layer 16 so that exposure of the object occurs through the backing plate 15 rather than through the dark slide 13. In such case as well as in the arrangement as shown in Fig. l, the object to be radiographed preferably rests on the dark slide (or on the backing plate in the reverse condition noted) provided the object is not heavy enough to cause mechanical damage. The advantage of this is that there seems to be less distortion of the image if the test object is as close as possible to the Xerographic plate. The prevention of undercutting is effected equally as well with the member if in the position shown inthe noted inverted condition as long as the positive potential is maintained on the dark slide 13.

It will be seen, therefore, that xeroradiography is improved by the rocess of this invention in that substantially all image undercutting caused by air ionization during radiation exposure is eliminated. This is effected by spacing a conductive metallic surface in parallelly disposed proximity to but out of contact with a xeroiii.

radiographic plate or element and applying and maintaining a positive electrode potential on said surface relative to said plate while exposing the plate to penetrating radiation. The maintenance of the positive potential on the metallic surface during exposure draws negative ions of ion pairs created in the air space between the plate and the metallic surface by the radiation and prevents discharge of the charged photoconductive surface by said ions thereby eliminating undercutting.

While specific embodiments and modes of practicing the invention have been described, variations within the scope of the appended claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact details shown or described.

What is claimed is:

1. That improvement in xeroradiography of objects to eliminate ionic undercutting of images of the radiographed object produced on a xeroradiographic plate comprising maintaining of a surface at a raised potential of the same polarity as the charge on the plate surface in spaced proximity to the charged photo-conductive surface of the xeroradiographic plate to create an electric field Whose lines of force extend substantially normal to the charged photoconductive surface while radiographing said object on said plate.

2. That improvement in xeroradiography to eliminate substantially all image undercutting caused by air ionization during radiation exposure comprising the steps of spacing a conductive metallic surface in parallelly disposed proximity to but out of contact with the charged photo-conductive surface of a xeroradiographic plate, and applying and maintaining a raised electric potential on said surface relative to said plate and of the same polarity as the polarity of the charge on the photoconductive surface while exposing the plate to penetrating radiation.

3. That improvement in xeroradiography of an object to eliminate ionic undercutting of the image of the radiographed object produced on a xeroradiographic plate comprising locating a metallic conductive sheet in parallel spaced proximity to the charged photo-conductive surface of a xeroradiographic plate, and applying and maintaining a positive electric potential on said sheet relative to said plate while radiographing said object on said plate.

4. That improvement in xeroradiography to eliminate ionic undercutting of the image of a radiographed object produced on an xeroradiographic plate comprising maintaining a positive potential on a conductive surface in close but parallel spaced proximity to a charged photoconductive surface of said radiographic plate while radiographing the object.

5. That improvement in xeroradiography to eliminate undercutting of the image of a radiographed object produced on an xeroradiographic plate comprising establishing an electric field whose lines of force extend substantially normal to a charged photo-conductive surface of said radiographic plate and maintaining said electric field while radiographing the object with penetrating radiation so that negative ions will be moved in the direction of said lines of force and away from said charged surface.

6. That improvement in xeroradiography to eliminate undercutting of the image of a radiographed object produced on a xeroradiographic plate comprising positioning a conducting surface in parallel, spaced proximity to an electrostatically charged photoconductive surface of the xeroradiographic plate and establishing an electric field between said conducting surface and said photoconductive surface Whose lines of force extend substantially normal to said photo-conductive surface, said clec .tric field being established by applying a positive potential to said conducting surface, and maintaining said field while radiographing the object so that negative ions occurring in proximity to the photo-conductive surface will be moved away from the latter to the conducting surface by the action of said. field.

7. The improvement of claim 6 wherein said positive potential is in the range of 700 to 1800 volts D. C.

8. Apparatus for eliminating undercutting of the image of a radiographed object comprising a xeroradiographic element, a conductive sheet, means for supporting the latter in parallel spaced proximity relative to the photoconductive surface of said element and means for applying a positive electric potential to said sheet relative to said element.

9. Apparatus for eliminating undercutting of the image of a radiographed object comprising a frame, a xeroradiographic element carried by the frame, a conductive sheet supported by the frame in parallel spaced proximity relative to said element, a conductive contact member in electric contact with said sheet, a conductive plate in electric contact with said xeroradiographic element, electrical connection between said contact member and a positive terminal of a D. C. source and electrical connection between said conductive plate and the negative terminal of said D. C. source.

10. Apparatus for eliminating undercutting of an image of a radiographed object comprising a frame of nonconduetive material, an xeroradiographic element including a backing sheet and a photo-conductive coating on said sheet, said element being supported by said frame, a conductive sheet supported by said frame in spaced parallelly disposed proximity relative to said photoconductive coating and in light excluding relationship thereto, a conductive metallic backing member in electrical contact with said backing sheet, a sprung contact member in electrical contact with said conductive sheet and an electric circuit including a resistor connected to said contact member and to the positive terminal of a source of direct current and an electrical connection from the negative terminal of said source to the conductive metallic backing member.

11. Apparatus for eliminating undercutting of the image of a radiographed object comprising a frame, means to support a xeroradiographic element in said frame, means to support a conductive sheet in said frame in parallel spaced proximity relative to a xeroradiographic element in position in said frame, a conductive contact member positioned and disposed to make electric contact with said sheet when a sheet is in position in said frame, a conductive plate positioned and disposed to make electric contact with said xeroradiographic element when a xeroradiographic element is in position in said frame,

electrical connection between said contact member and a positive terminal of a D. C. source and electrical connection between said conductive plate and the negative terminal of said D. C. source.

12. Apparatus for eliminating undercutting of the image of a radiographed object comprising a frame, a xeroradiographic element carried by the frame, a conductive sheet supported by the frame in parallel spaced proximity relative to said element, a conductive contact member in electric contact with said sheet, a conductive plate in electric contact with said Xeroradiographic element, electrical connection between said contact member and a terminal of a D. C. source and electrical connection between said conductive plate and the opposite polarity terminal of said D .C. source, said electrical connections between said D. C. source and said contact member and said conductive plate being arranged to apply a D. C. potential to said contact member of the same polarity as the polarity of charge on said xeroradiographic element while said element is in a charged condition for exposure.

13. Apparatus for eliminating undercutting of the image of a radiographed object comprising a frame, a charged xeroradiographic element carried by the frame, a conductive sheet supported by the frame in parallel spaced proximity relative to said element, and means for applying an electric field between said conductive sheet and the xeroradiographic element, said field being applied to create on the conductive sheet a potential of the same polarity as the polarity of charge on the xeroradiographic element.

14. Apparatus for eliminating undercutting of the image of a radiographed object comprising a frame, a Xeroradiographic element positioned in said frame, a sheet supported by said frame in parallel closely spaced proximity to said element but out of contact therewith, and means for applying an electric field between said sheet and the xeroradiographic element, said field being arranged to apply a positive polarity potential to said sheet.

References Cited in the file of this patent UNITED STATES PATENTS 2,004,453 Watanabe June 11, 1935 2,257,774 Von Ardenne Oct. 7, 1941 2,372,465 Whittaker Mar. 27, 1945 2,440,640 Marton Apr. 27, 1948 2,666,144 Schaifert et al. Jan. 12, 1954 

